Silver halide photography usually involves the exposure of silver halide with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. Thus, when silver halide is to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element or infrared radiation in an infrared-sensitive element, a spectral sensitizing dye is required. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds) that are adsorbed to the silver halide. They absorb light or radiation of a particular wavelength and transfer the energy to the silver halide to form the latent image, thus effectively rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity.
Silver halide is often spectrally sensitized with certain trimethine cyanine dyes, also called carbocyanines. In order to provide high sensitivity and also to provide sensitivity at the proper wavelength of radiation, it is often desirable for the sensitizing dye molecules to aggregate when used to sensitize silver halide emulsions. Carbocyanines that form such aggregates are well-known in the art. One such known dye has the formula: ##STR2##
Dyes, such as the one shown above, although they form an aggregate such as a J-aggregate so as to provide high photographic sensitivity and good spectral absorbance characteristics, also suffer from the disadvantage that they tend to be retained in the layers of the photographic material after processing, leading to undesirable dye stain.
The prior art has generally used stain-reducing agents, such as bis-triazine stilbene compounds known as optical brighteners, in order to reduce this retained dye stain. These compounds, however, are expensive and can be difficult to incorporate in the hydrophilic layers in photographic materials.
On the other hand, dyes, such as: ##STR3## tend to have low stain, but do not aggregate to an extent sufficient to provide the desired spectral sensitivity to silver halide emulsions.
It is thus an object of this invention to spectrally sensitize silver halide photographic compositions with a carbocyanine dye that provides high sensitivity, yet also provides low stain while avoiding the necessity of using stain-reducing agents.